Tensioning devices, such as hydraulic tensioners, are used as a control device for a power transmission chain, or any similar power transmission device, as the chain travels between a plurality of sprockets. Generally, it is important to impart and maintain a certain degree of tension to the chain to prevent noises, slippage, or the unmeshing of teeth in cases of a toothed belt.
Prevention of such slippage is especially important in the case of a chain driven camshaft in an internal combustion engine. In such an installation, chain slippage may throw off the camshaft timing by several degrees, possibly rendering the entire engine inoperative or causing damage. In the harsh environment in which an internal combustion engine operates, however, chain tension may vary between excessively high or low levels as a result of the wide variations in temperature as well as differences between the co-efficients of linear expansion among the various engine parts, including the chain and tensioner. Wear to chain components during prolonged use, moreover, can result in a decrease in chain tension. Thus, it is also necessary to provide some measures to remove excessive tensioning forces on the tight side of the chain as well as to ensure necessary tensioning forces are imparted on the slack side of the chain. Cam shaft and crank shaft induced torsional vibrations, furthermore, may cause belt tension to vary considerably. This tension variation may result in chain elongation, possibly rendering the entire engine inoperative.
One example of a device used to control tension in a wrapped power transmission device is described in Kimura et al., U.S. Patent No. 4,708,696. Kimura et al. discloses a hydraulic tensioner having a piston chamber and a piston biased by a spring in a protruding direction therefrom. A rod extends from the piston and the piston chamber to impart tension to a chain. The piston functions to separate the piston chamber into a first hydraulic fluid chamber and a second hydraulic fluid chamber. The first hydraulic fluid chamber is connected to the second hydraulic fluid chamber by a check valve mounted on the piston. The check valve permits fluid to freely flow from the first chamber to the second chamber as the piston protrudes the rod from the piston chamber. Thus, the hydraulic pressure of the fluid and the force exerted by the spring cause the rod to impart tension to a chain. On the other hand, chain tension may become greater than the combined force of the spring and hydraulic pressure. At this point, chain tension will tend to force the piston in the reverse direction, inwardly towards the piston chamber. The check valve, however, restricts the reverse flow of fluid from the second chamber to the first chamber. Due to the incompressibility of hydraulic fluid, the piston is thus limited in its reverse movement within the piston chamber. In such a fashion, the tensioner achieves a so-called no-return function, i.e., movements are easy in one direction but difficult in the reverse direction. A small clearance between the piston and the piston chamber wall, however, permits limited fluid flow between the second chamber to the first chamber. This allows the piston some limited movement in the reverse direction, i.e., to retract somewhat. This feature provides for excessive tensioning forces to be attenuated. Kimura et al. further discloses a system of passages connected to a diaphragm which prevent air from entering the piston chamber.
One of the problems associated with the use of such a hydraulic tensioner, however, is the venting of entrapped air from the piston chamber while the tensioner is operating. Air may enter the hydraulic fluid system and become entrapped within the piston chamber, on occasion, due to the variations in hydraulic fluid pressure or system leakage, for example. Venting such air from the piston chamber is important because air is much more compressible than hydraulic fluid. Thus, air, due to its compressibility, permits the piston to be forced further back into the piston chamber. Less tension is thereby maintained and possible chain slippage may result.
Accordingly, it is an object of the present invention to provide a tensioner for chain, belt or similar wrapped power transmission devices which can maintain a substantially constant tensioning force.
It is a further object of the present invention to provide a hydraulic tensioner which readily permits the venting of any air out from the tensioner.